Our long-term objective is to develop a detailed understanding of the physiological functions of ion transporters involved in secretion and absorption of ions in the gastrointestinal tract and in renal control of acid/base and electrolyte homeostasis. In working toward this goal, we are using gene targeting technology to examine the in vivo functions of specific transporters. We have already developed null mutant mice for the NHE1, 2, and 3 Na/H exchangers, the colonic and gastric H,K-ATPases (cHKA and gHKA), the thiazide-sensitive NaCl cotransporter, and the basolateral NaK2Cl cotransporter (NKCC1). Aim 1 of this proposal is to develop null mutant mice for the NHE4 Na/H exchanger, the AE2 and DRA C1/HCO3 exchangers, the NBC1 Na/HCOs cotransporter, and the C1C-2 Cl channel, and to perform the initial analyses of their phenotypes. In aim 2, the transport mechanisms underlying gastric HC1 and KC1secretion and mucosal protection will be analyzed. These studies will test the hypotheses that C1C-2, AE2, and NHE4 are essential for HC1secretion by the gastric parietal cell;that NHE2 functions in mucosal protection and in maintaining parietal cell viability during acid secretion;and that NKCC1 is required for maximum KC1 secretion by gastric glands. In aim 3, the transport mechanisms underlying absorption and secretion in the intestinal tract will be analyzed. These studies will test the functions of: DRA as the apical Cl/HCOs exchanger mediating Cl absorption;AE2 as the basolateral C1/HCO3 exchanger mediating HCC>3 absorption;NHE1and NKCC1 in toxin-stimulatedsecretion;NHE2 in blunting the severity of diarrheal states;and cHKA in K recycling in the colon and in providing compensation for diarrheal states. In aim 4, the function and relative importance of specific transporters in renal function will be analyzed. In these studies we will: analyze the renal phenotypes of mice lacking NKCC1, AE2, and NHE2;test the functions of cHKA and gHKA in HCO3 reabsorption in the collecting duct and in urinary potassium conservation during potassium depletion;and analyze the relative importance of NBC1 and NHE3 in HCOa reabsorption in the proximal tubule and in acid-base homeostasis. These studies will substantially expand our understanding of the specific functions of individual transporters and how they work together to mediate transepithelial ion transport processes in vivo.